Temporal and sex-specific variability in Rhinoceros Auklet diet in the central California Current system

•We used stable isotopes to evaluate diet of Rhinoceros Auklets during two years.•Seasonally-important prey species were northern anchovy and juvenile rockfish sp.•Adult trophic level was greater during chick-rearing than during incubation.•Sex-specific differences in adult diet occurred during chick-rearing 2012.•Sex-specific differences may have been related to female energetic constraints.

We used stable isotopes (δ15N and δ13C) and compared prey provided to chicks by each sex to evaluate seasonal and sex-specific diets in Rhinoceros Auklets (Cerorhinca monocerata) in the central California Current system during 2012–2013. Mixing models indicated northern anchovy (Engraulis mordax) were important prey for adults during fall/winter and juvenile rockfishes (Sebastes spp.) were important prey during incubation both years. Adult trophic level increased between incubation and chick-rearing periods in both years. During 2012, δ15N and δ13C of chick-rearing males and females differed significantly; mixing models indicated that females ate more Pacific saury (Cololabis saira) and less market squid (Doryteuthis opalescens) than males. Likewise, females delivered significantly more Pacific saury and less market squid to chicks than males during 2012. Chick growth (g d− 1) and chick survival to fledging were significantly lower during 2012 than 2013, likely because chicks were fed lesser quality prey or fed less frequently in 2012. Lesser body mass of females during incubation in 2012 indicated sex-specific diet differences may have been related to female energetic constraints. The observed variability in Rhinoceros Auklet diet underscores the importance of managing multiple prey populations in this system so that generalist predators have sufficient resources through changing conditions.